Transcoder and imaging apparatus for converting an encoding system of video signal

ABSTRACT

The time required to transcode an encoded image signal to an image signal encoded by a different encoding method is reduced.  
     Disclosed herein is a transcoder comprising: a picture selector  4  which extracts specific types of pictures (I- and P- pictures) from a first image signal (MPEG2 stream) and generates a subset of the first image signal; a first decoder  5  which decodes the subset image signal; and a first encoder  7  which encodes the decoded image signal to a second image signal (MPEG4 stream). The picture selector  4  uses the extracted pictures to generate the subset image signal with a reduced effective length.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transcoder for converting the codingformat of an image signal and to an imaging apparatus and signalprocessor using this transcoder.

2. Description of the Related Art

3. Prior Art

To record moving image signals on recording media or transmitting themover the Internet, radio and other communication media, techniques forcompressing and encoding moving image signals by MPEG and othercompressing algorithms have recently been used in various fields. Forexample, MPEG2-Video (ISO/IEC13818-2) is used for recording to andplaying back from DVDs. In addition, MPEG4-Video (ISO/IEC14496-2) isused in radio communication and Internet applications. Including them, avariety of encoding methods have been proposed. With the diversificationof encoding methods, however, it has become necessary to transcode dataamong these formats and improve the transcoding capability. Inparticular, if the time required for the transcoding process is reduced,the user side is given raised convenience.

A technique disclosed in Japanese Patent Laid-open No. 2002-152755concerns improvement in the transcoding speed. This technique “reducesthe temporal resolution of an input video stream by thinning outbidirectional predictive frames and reduces the spatial resolution byreducing the picture size of the temporal resolution-lowered videostream without inversely transforming transform coefficients” aimed at“shortening the transcoding time required to convert the video data to adifferent kind of video data by entirely eliminating the necessity ofinverse transformation of coded transform coefficients in the frequencydomain”.

BRIEF SUMMARY OF THE INVENTION

According to the technique described in Japanese Patent Laid-open No.2002-152755, the transcoding time can be reduced by eliminating thedecoding process (i.e., inverse transformation of transformcoefficients). However, in such a transcoder as to be configured toinclude a decoder for decoding an MPEG stream for display as an imagesignal, combined with an encoder for encoding/compressing the inputimage signal into an MPEG stream, eliminating the inverse transformationof transform coefficients does not substantially reduce the timerequired for transcoding if transcoding is made at a rate of one pictureper frame period.

It is an object of the present invention to solve the above-mentionedproblem and provide improved usability to the user by reducing the timerequired for transcoding.

To solve the above-mentioned problem, according to an aspect of thepresent invention, there is provided a transcoder which inputs a firstimage signal encoded by a first compressing and encoding method andtranscodes the first image signal to a second image signal encoded by asecond compressing and encoding method. The transcoder comprises: apicture selector which generates a subset image signal of the firstimage signal by extracting pictures of one ore more specific types inframes or fields from the first image signal; a first decoder whichdecodes the subset image signal generated by the picture selector; and afirst encoder which encodes the decoded image signal by a secondcompressing and encoding method. The picture selector uses the extractedpictures to generate the subset image signal with a shorter effectivelength.

According to another aspect of the present invention, there is provideda transcoder which reads out a first image signal encoded by a firstcompressing and encoding method from a recoding medium and transcodesthe first image signal to a second image signal encoded by a secondcompressing and encoding method. The transcoder comprises: an interfacesection which generates a subset image signal of the first image signalby extracting pictures of one or more specific types in frames or fieldsfrom the first image signal; a first decoder which decodes the subsetimage signal generated by the interface section; and a first encoderwhich encodes the decoded image signal by a second compressing andencoding method. The interface section uses the extracted pictures togenerate the subset image signal with a reduced effective length.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

These and other objects, features and advantages of the presentinvention will become more readily apparent from the following detaileddescription when taken in conjunction with the accompanying drawingwherein:

FIG. 1 is a block diagram of a transcoder, according to a firstembodiment of the present invention;

FIG. 2 shows a configuration of the picture selector in FIG. 1;

FIG. 3 is a imaginary diagram showing an example of a picture selectingscheme in FIG. 1;

FIG. 4 is a diagram showing a preferable picture extracting condition inFIG. 1;

FIG. 5 is a block diagram of another transcoder according to a secondembodiment of the present invention;

FIG. 6 shows an example of a file configuration on a recording medium 1in FIG. 5; and

FIG. 7 is a block diagram of an imaging apparatus according to a thirdembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a block diagram of a transcoder according to a firstembodiment of the present invention. In FIG. 1, a stream, read out froma recording medium 1, is transcoded by a transcoder 3 and output from anoutput terminal 50. The transcoder 3 comprises a picture selector 4, anMPEG2 decoder 5, a frame memory 6 and an MPEG4 encoder 7. In addition,the signal accumulated in the frame memory 6 is displayed by a display 9via a display circuit 8.

In the transcoder shown in FIG. 1, a MP@ML (main profile at main level)MPEG2 stream (hereafter denoted simply as an MPEG2/MP@ML stream) istranscoded to a SP (simple profile) MPEG4 stream (hereafter denotedsimply as an MPEG4/SP stream) and output.

Specifically, it is assumed in the following operational descriptionthat a 30 frames/sec MPEG2/MP@ML stream is transcoded to a 10 frames/secMPEG4/SP stream. Firstly, an MPEG2 stream stored on the recording medium1 such as a DVD is read out by an interface 2 and supplied to thepicture selector 4. The picture selector 4 searches the incoming MPEG2stream for picture headers and extracts pictures of a specific kind inframes or fields so as to generate an MPEG2 stream whose picturepopulation is reduced to a third. How the picture selector 4 isconfigured will be described later in detail. The MPEG2 stream, aftergiven extraction processing, is output and supplied from the pictureselector 4 to the MPEG2 decoder 5. The MPEG2 decoder 5 converts theinput MPEG stream to a digital video signal containing pixel value databy decoding the input MPEG stream. Pixel value data is accumulated inthe frame memory and, according to a display sync signal, read out tothe display section for output on the display 9. Concurrently, thedigital video signal is supplied from the frame memory 6 to the MPEG4encoder 7 which in turn encodes the signal to an MPEG4 stream andoutputs it to the output terminal 51.

FIG. 2 shows a configuration of the picture selector 4 in FIG. 1. Thepicture selector 4 has a separator 41, a buffer memory 42 and a buffermemory readout circuit 43. The MPEG2 stream has audio, video andadditional data and others multiplexed thereinto. The separator 41generates ESs (Elementary Streams) by separating the MPEG2 streamaccording to data type and performing system decode. A video ES isaccumulated in the buffer memory 42. Provided with a picture head searchfunction, the buffer memory readout circuit 43 sequentially searches thebuffer memory 42 to detect each picture head and determines the type ofthe associated picture. If a determined picture type is the type ofpictures to be extracted, data is read out until just before the nextpicture head. Extraction of pictures of specific picture types isrealized through this procedure.

FIG. 3 is an imaginary diagram showing an example of how pictures areselected in the present embodiment. The MPEG2 stream comprises threetypes of pictures: I-pictures coded within frames, P-pictures predictedand coded with reference to past frames and B-pictures predicted andcoded with reference to both past and future frames. FIG. 3A shows avideo ES 300 stored in the buffer memory 32. A video ES 301 shown inFIG. 3B is a result of the video ES 300 read out from the buffer memory42 and given picture selection by the readout circuit 43. Of the inputvideo ES, only I-pictures and P pictures are extracted whereasB-pictures are removed. If the video ES 300 has a picture sequence<IBBPBBPBBPBBPBBIBB . . . >, the video ES 301 has a picture sequence<IPPPPPI . . . >.

Firstly, the buffer memory readout circuit 43 detects a picture header302 in the video ES 300, judges that the picture is a I-picture which isto be extracted, and starts reading out data. Then, the readout circuit43 detects the next picture header 303, judges that the picture is aB-picture which is to be removed, and stops reading out data just beforethe picture header 303. The readout circuit 43 resumes header search anddetects picture headers 304 and 305. The readout circuit 43 detects thatthe picture following the picture header 305 is a P-picture which is tobe extracted, and starts reading out data. By repeating this procedure,only I-pictures and P-pictures are extracted from the video ES 300 togenerate the video ES 301.

As apparent in FIG. 3, the video ES 301 is configured in such a mannerthat I-picture data is immediately followed by the subsequentlyextracted P-picture data. Therefore, as a subset of the video ES signal300, the video ES 301 is shorter than the video ES 300. Note thatalthough not shown in the figure, the subset video signal may also bearranged so as to contain a space for each non-extracted B-picture. Thisdoes not change the effective length since the spaces can be skipped inthe subsequent signal processing (decoding).

The MPEG2 decoder decodes the supplied MPEG2 stream and stores it in theframe memory 6 so that the decoded video signal may be displayed. Thestored video signal is retained until displayed. In order to prevent theframe memory from overflowing or underflowing, the decoding speed mustbe equal to the display speed. Accordingly, increasing the total numberof pictures increases the number of pictures to be displayed. The timerequired for recording becomes longer in proportion to the total numberof pictures.

Assume that the video ES 300 is to be transcoded in a conventionalmethod. In this case, after the non-thinned out picture sequence<IBBPBBPBBPBBPBBPBBI . . . > is decoded, B-pictures are removed tosupply a stream <IPPPPI . . . > to the MPEG4 encoder 7 for encoding.Accordingly, the time required for transcoding equals the time requiredto decode and display the non-thinned out video ES 300 which containsall pictures.

According to the present invention, the above-mentioned transcoding isperformed after a video ES 301, a subset of the video signal ES 300, isgenerated by removing B-pictures from the video ES 300. The MPEG decoder5 decodes the video ES 301, that is, only the pictures <IPPPPI . . . >and directly supplies the decoded video signal to the MPEG4 encoder. Thetotal number of pictures contained in the video ES 301 is reduced to athird by the picture extraction circuit 4 as compared with the totalnumber of pictures contained in the video ES 300. Thus, since the timerequired for transcoding is reduced to a third as compared with thatrequired to decode and display the video ES 300, the transcoding timecan be reduced remarkably. In this case, pictures are displayed at thetriple speed via the display unit 8 as compared with the display speedtaken when the stream is played back without removing pictures.

FIGS. 4A to 4C show a preferable picture extracting condition which canbe employed in the picture selector 4. In the MPEG prediction and codingmethod, reference is required between pictures for inter-picture motioncompensation. For a pre-extraction stream as shown in FIG. 4A, forexample, a B-picture 102, is predicted and coded with reference to aP-picture 101 and a P-picture 103. FIGS. 4B and 4C are examples ofstreams derived as results of extraction by the picture selector 4. InFIG. 4B, the I-pictures and P-pictures are extracted whereas theB-pictures are removed. This is one of the preferable extracting schemessince each extracted P-picture can refer to other extracted pictures. Inthe case of the extraction scheme shown in FIG. 4C, however, extractedB-pictures 111 and 113 and P-picture 114 cannot refer to a removedP-picture 112 which should be referred to. This makes it impossible todecode these extracted pictures. That is, the picture extraction schemeof the picture selector 4 must be designed so that each extractedpicture can refer to another extracted picture.

As other preferable extraction schemes, extracting only I-pictures froma video stream comprising I-, P- and B-pictures, extracting I-picturesfrom a video stream comprising I- and P-pictures and extracting specificI-pictures from a video stream comprising I-pictures are alsoapplicable. Further, such a configuration is possible as to allow theuser to specify what pictures are to be extracted.

The aforementioned embodiment is also characterized by the processingspeed of the picture selector 4. As described earlier, when the stream(video ES) 300 in FIG. 3A is thinned to the stream 301 in FIG. 3B, thetotal number of pictures contained in the stream reduces to a third.Because of this reduced number of pictures, the time required for thesubsequent decoding can be shortened. For the picture selector 4 toperform extraction and removal without delay, however, data must besupplied to the picture selector 4 at a higher rate than the decodingrate. In the above-mentioned case where B-pictures are removed from astream having a picture sequence <IBBPBBPBBPBBPBBI . . . > to generate astream <IPPPPI . . . >, the stream must be supplied to the pictureselector 4 at a bit rate three times as fast as the decoding bit rate ifthe I-, P- and B-pictures has the same amount of code per picture.Otherwise it is not possible for the picture selector 4 to performextraction and removal without delay. Generally, however, sinceI-pictures have the largest amount of code, followed by P-pictures andthen B-pictures, the stream supply bit rate must not be three times asfast as the decoding bit rate. Since it is specified that an MPEG2/MP@MLmay have a bit rate of up to 15 Mbits/sec, it is sufficient to supplythe stream to the picture selector 4 at 45 Mbits/sec, three times asfast as the maximum bit rate.

As an example, assume that the recording medium 1 is a DVD-ROM. In thiscase, since data can be input to the picture selector 4 via theinterface section 2 at a bit rate of up to 160 Mbits/sec, it is possibleto supply MPEG2 stream data to the picture selector 4 at a sufficientlyhigh bit rate. In addition, since the readout circuit 43 can performheader search processing at a maximum bit rate of about 1 Gbits/sec ifit operates in 16 bits at 81 MHz, the readout circuit 43 can easilyafford to perform header search processing on the data which is input atthe above-mentioned bit rate. The same header search processing can alsobe implemented by software. For example, if the CPU operates at 200 MHzand 30 cycles are spent to process each word, header search processingcan be executed at a maximum bit rate of 106 Mbits/sec, which issufficiently high for the input bit rate.

Then, FIG. 5 shows a block diagram of another transcoder according to asecond embodiment of the present invention. This embodiment differs fromthe first embodiment (FIG. 1) in that to extract and remove picturesfrom a stream which is read out from a recording medium 1, an interface2 is used instead of the picture selector 4 of the first embodiment. Onthe recording medium 1, a stream entered from an input terminal 51 isstored via the interface 2.

If the recording medium 1 is a DVD or the like, a management informationfile is stored together with an MPEG2 stream. A management informationfile is created by the interface 2 when a stream file, entered from theinput terminal 51, is to be stored on the recording medium 1. In themanagement information file, the picture type, storage location and sizeof each picture are recorded. By referring to this managementinformation file, the interface 2 identifies the locations of therespective pictures to be extracted. Accordingly the interface 2extracts these pictures from the stream and supplies them to an MPEG2decoder 5 where transcoding is done by the same process as in the firstembodiment.

FIG. 6 shows the file configuration prescribed in the DVD VideoRecording specification. On the recording medium, the DVD_RTAV directory200 exists with the hierarchically lower VR_MANGR.IFO file 201 andVR_MOVIE.VRO file 202. VR_MOVIE.VRO 201 stores MPEG2 stream data whichis divided into small units called VOBUs (Video Object Units). Each VOBUhas one I-picture. VR_MANGR.IFO 202 is a management information filewhere the location and size of the I-picture in each VOBU are recordedin the form of a table for use in fast forward, backward and otherspecial play modes. Therefore, when I-pictures are to be extracted, theinterface block 2 can refer to VR_MANGR.IFO 201 to recognize thelocation and size of each I-picture. Each I-picture can be extracted byreading out as large data from the recognized location as the recognizedpicture size.

According to this embodiment, since the video signal is downscaled to asubset in the stage of extraction/readout from the recording medium 1,the time required for the subsequent processing can be shortened.

Although it is assumed in the above-mentioned example that the recodingmedium conforms to the DVD Video Recording specifications, thisembodiment is also similarly applicable to other recording media ifmanagement information indicating the type and location of each pictureis recorded thereon.

Note that if the recording medium 1 is a removal recording medium and astream is written onto the medium by using another apparatus, there is apossibility that the management file may not exist thereon. By takinginto such a case into consideration, this embodiment may also beprovided with such a function as to analyze the stored stream read outby the interface block 2 and create a management information file forstorage on the recording medium.

FIG. 7 is a block diagram of an imaging apparatus according to a thirdembodiment of the present invention. This embodiment is an imagingapparatus where the transcoder shown in FIG. 1 as the first embodimentis used. Besides the configuration of the first embodiment, this imagingapparatus 30 comprises a camera unit 10 and a user interface 20. Thecamera unit 10 is composed of an image pickup block 11 and an MPEG2encoder 12.

If it is demanded by the user to record MPEG2 moving images on therecording medium 1, the user interface 20 issues a recording instructionto start the image pickup block 11 in the camera unit 10 generating adigital video signal. The MPEG2 encoder 12 generates an MPEG2 stream byencoding the digital video signal. Then, the MPEG2 stream is recorded onthe recording medium 1 via the interface 2.

In addition, if it is demanded to transcode the MPEG2 stream recorded onthe recording medium 1 to an MPEG4 stream, the user interface 20 issuesa transcode instruction. The same transcoding process as in the firstembodiment is started by this instruction to output an MPEG4 stream.

That is, in this imaging apparatus described as the third embodiment, itis possible not only to pick up images and record them as a high imagequality MPEG2 stream but also to convert it to a low bit rate MPEG4stream for output to the outside.

Although the imaging apparatus in FIG. 7 uses the transcoder shown inFIG. 1 as the first embodiment, it is also possible to configure theimaging apparatus by using the transcoder shown in FIG. 5 as the secondembodiment. In addition, although the camera unit 10 has the imagepickup block 11 therein, it is possible to modify the configuration insuch a manner that a video signal receiver is connected in order toinput video signals from the outside. It is also possible to modify theconfiguration so as to connect an MPEG2 stream receiver in order toinput MPEG2 streams from the outside.

Although the first to third embodiments have been described on theassumption that the compressing/encoding format is transcoded from MPEG2to MPEG4, the present invention is not limited to this transcodingscheme but can also be applied to between other compressing/encodingformats.

Further, a transcoder according to the present invention can be appliednot only to such an imaging apparatus as mentioned above but also tosuch apparatus as a receiver (set-top box) which receives a video signal(stream) and transcodes it before outputting it to a display, an imageoutput apparatus (display) which transcodes an input image signal beforedisplaying it, an image recording apparatus which transcodes an inputimage signal before recoding it onto a recording medium (hard disk,optical disk or the like) and a home server which is connected tovarious image apparatus to supply image signals.

According to the present invention, it is possible to provide atranscoder and imaging apparatus improved in usability for the user.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentedembodiments are therefore to be considered in all respects asillustrative and not restrictive. The scope of the invention beingindicated by the appended claims rather than by the followingdescription and range of equivalency of the claims are thereforeintended to be embraced therein.

1. A transcoder which inputs a first image signal encoded by a firstcompressing and encoding method and transcodes the first image signal toa second image signal encoded by a second compressing and encodingmethod, said transcoder comprising: a picture selector which generates asubset image signal of the first image signal by extracting pictures ofone or more specific types in frames or fields from the first imagesignal; a first decoder which decodes the subset image signal generatedby the picture selector; and a first encoder which encodes the decodedimage signal by a second compressing and encoding method; wherein thepicture selector uses the extracted pictures to generate the subsetimage signal whose effective length is reduced.
 2. A transcoder whichreads out a first image signal encoded by a first compressing andencoding method from a recoding medium and transcodes the first imagesignal to a second image signal encoded by a second compressing andencoding method, said transcoder comprising: an interface section whichgenerates a subset image signal of the first image signal by extractingpictures of one or more specific types in frames or fields from thefirst image signal; a first decoder which decodes the subset imagesignal generated by the interface section; and a first encoder whichencodes the decoded image signal by a second compressing and encodingmethod; wherein the interface section uses the extracted pictures togenerate the subset image signal whose effective length is reduced.
 3. Atranscoder according to claim 1, wherein the subset image signal has theextracted pictures arranged sequentially therein and the effectivelength of the subset image signal is reduced.
 4. A transcoder accordingto claim 2, wherein the interface section extracts and reads outpictures of one or more specific types by referring to managementinformation recorded along with the first image signal on the recordingmedium.
 5. A transcoder according to claim 1, wherein the pictureselector performs picture extraction in such a manner that eachextracted picture can refer to another extracted picture for motioncompensation.
 6. A transcoder according to claim 1, wherein: the firstcompressing and encoding method is an MPEG2 method and the secondcompressing and encoding method is an MPEG4 method; and the pictureselector generates the subset image signal by extracting I-pictures andP-pictures.
 7. A transcoder according to claim 1, wherein the pictureselector allows the user to specify what types of pictures are to beextracted.
 8. A transcoder according to claim 1, wherein a bit rate ofthe first image signal supplied to the picture selector is set so as tocompensate for an amount of code of the pictures which are not extractedwhen the subset image signal is generated.
 9. A transcoder according toclaim 1, further comprising: a frame memory for storing the image signaldecoded by the first decoder; and a display section which reads out theimage signal from the frame memory and outputs the image signal to adisplay unit; wherein images being transcoded are displayed on thedisplay unit.
 10. An imaging apparatus using the transcoder according toclaim 1, said imaging apparatus comprising: an image pickup sectionwhich picks up an object; a second encoder which, by the firstcompressing and encoding method, encodes the first image signal suppliedfrom the image pickup section; and a recording and reproducing sectionwhich records and reproduces the first image signal encoded by thesecond encoder to and from a recording medium, wherein the first imagesignal reproduced from the recording medium is supplied to thetranscoder.
 11. An imaging apparatus according to claim 10, furthercomprising: a receiver which receives an image signal from the outside;wherein the second encoder encodes the image signal supplied from thereceiver by the first compressing and encoding method.
 12. An imagingapparatus according to claim 10, further comprising: a receiver whichreceives the first image signal encoded by the first compressing andencoding method from the outside; wherein the recording and reproducingsection records and reproduces the first image signal supplied from thereceiver to and from the recording medium.
 13. An imaging apparatusaccording to claim 10, wherein the recording and reproducing apparatusgenerates management information from the first image signal recorded onthe recording medium and records the management information on therecording medium.
 14. A signal processor using the transcoder accordingto claim 1, wherein said signal processor inputs a first image signalencoded by a first compressing and encoding method, transcodes the firstimage signal to a second image signal encoded by a second compressingand encoding method and outputs the second image signal to externalequipment.